The present invention relates to ink jet printing apparatus and is concerned, more particularly, with the priming of the printhead(s) in such apparatus.
An ink jet printer may be of the "continuous stream" or the "drop-on-demand" type. In the continuous stream type of printer, ink is emitted continuously from one or more orifices in a printhead, producing droplets which are deflected as necessary so that they are deposited either in a specific location on a recording member or, if not required for printing, in a gutter from where they are recirculated. In the drop-on-demand type of printer, ink is contained in a plurality of channels in a printhead and energy pulses are used to cause the droplets of ink to be expelled, as required, from orifices at the ends of the channels and directed towards a recording member.
In a thermal ink jet printer, those energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As a vapour bubble grows in any one of the channels, ink bulges from the channel orifice until the current pulse has ceased and the bubble begins to collapse. At that stage, the ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel and towards the recording medium. The channel is then re-filled by capillary action, which in turn draws ink from a supply container.
It is usually necessary to prime a printhead of an ink jet printer before use, to remove air and ensure that the printhead is full of ink. Priming may, for example, be carried out by applying suction to the ink ejecting orifice(s) to draw ink into the printhead. Alternatively, ink can be forced into the printhead under pressure.
U.S. Pat. No. 4,734,719 describes an ink jet printer in which a capping device is provided to apply suction to the printhead orifices to recover the discharge function of the printhead after a period of non-use. In that printer, the ink channels within the printhead communicate with, and receive ink from, a sub-tank which in turn is supplied with ink from a remote main tank. Air collects in the sub-tank and is removed by applying suction to the sub-tank before suction is applied to the printhead orifices. Suction is applied to the sub-tank via a plurality of suction tubes provided specifically for that purpose. Another printer in which the printhead is primed by applying suction to the printhead orifices is described in U.S. Pat. No. 4,853,717. In that printer, the printhead is part of a cartridge which also contains a reservoir of ink.
U.S. Pat. No. 4,575,738 describes an ink jet printer in which pressurized air is used to deliver ink from a remote supply to the printhead via an ink chamber which forms part of the printhead module. Any entrained air in the ink is separated out and trapped in the ink chamber. To remove the trapped air, a purging vent in the chamber is opened and the air is then forced out through the vent by delivering ink to the chamber. Another arrangement for removing air from the ink chamber of a printhead while printing is in progress is described in U.S. Pat. No. 4,679,059.
U.S. Pat. No. 4,591,873 to McCann et al discloses an ink jet printing apparatus with an orifice cleaning system including cooperative elements which apply varying pressure differentials across an orifice plate to oscillate ink into and out of the orifices. The pressure differentials may be implemented by varying ink impedance cross-flow through a printhead.
U.S. Pat. No. 4,837,585 to Williams et al discloses an ink jet printer having an improved system for reducing pressure variations. A damping system reduces ink pressure transients within a printhead means and includes a damping chamber for a gas-over-ink region coupled to an ink return conduit and a printer subsystem for periodically introducing gas into the ink return conduit.
U.S. Pat. No. 4,518,974 to Isayama discloses an ink jet air removal system which detects the presence of air bubbles in ink and removes these bubbles by pulling an ink-air boundary into an ink chamber thereby transferring the air bubbles to the air.
The present invention relates to ink jet printers of the type in which the printhead has an associated ink reservoir through which ink is supplied to the printhead from a remote supply tank and in which air collects, for example by separating out from the ink before the ink enters the printhead. Conventionally, in a printer of that type, the reservoir is provided with a vent through which air can be removed, particularly when the printhead assembly is being primed. In some printers the vent is connected via a return line to the remote supply tank.